In general, the afterglow time of a phosphor is extremely short, and the emission of a phosphor rapidly decays when external excitation is ceased. However, in rare cases, even after the cessation of external excitation, some phosphors maintain afterglow of a level perceivable with the naked eye for a considerably long period of time (from a few tens of minutes to a few hours); such a phosphor is referred to as a phosphorescent phosphor or a phosphorescent material in discrimination of such a phosphor from common phosphors.
Examples of the known phosphorescent phosphors include: sulfide phosphors such as CaS:Bi (purple-blue emission), CaSrS:Bi (blue emission), ZnS:Cu (green emission) and ZnCdS:Cu (yellow-orange emission). Any of these sulfide phosphors suffers from a problem such that the phosphors are chemically unstable or poor in light resistance. Even when the zinc sulfide phosphorescent phosphors are used for luminous watches, there is a practical problem such that the afterglow time allowing the naked eye to perceive the time shown by the watches is approximately 30 minutes to 2 hours.
For the purpose of solving these problems, there has been proposed an aluminate phosphorescent phosphor being chemically more stable and more excellent in light resistance, and having an afterglow lasting for a longer period of time than the aforementioned sulfide phosphors. Specifically, there has been proposed a phosphorescent phosphor comprising, as a matrix, a compound represented by MAl2O4, wherein M represents at least one or more metal elements selected from the group consisting of calcium, strontium and barium (see, for example, Patent Literature 1).
Additionally, there have been proposed phosphorescent phosphors adaptable to various use conditions such as the following: phosphorescent phosphors having a high afterglow luminance in applications for low illumination intensity environments such as indoor type safety signs for escape guidance (see, for example, Patent Literature 2 and Patent Literature 3); and a phosphorescent phosphor having a high afterglow luminance after a long period of time under the conditions that the excitation by the sunlight is provided in the daytime up until sunset such as outdoor type safety signs (see, for example, Patent Literature 4). There has also been proposed a phosphorescent phosphor paying attention to the afterglow characteristics at high temperatures (see, for example, Patent Literature 5).
However, a further practical improvement of the afterglow luminance has been demanded.